This invention relates to a practical method for accurate and high-resolution measurements of the orthogonally polarised reflected Bragg wavelengths from fibre Bragg grating (FBG) sensors with birefringence. A fibre Bragg grating (FBG) is a permanent, perodic refractive index modulation in the core of a single-mode optical silica glass fibre over a length of typically 1-100 mm, formed by transversly illuminating the fibre with a periodic interface pattern generated by ultra-violet laser light, e.g. from an Eximer laser, either by using a two-beam interferometer, as disclosed by G. Meltz et.al. in [xe2x80x9cFormation of Bragg gratings in optical fiber by a transverse holographic method,xe2x80x9d Opt. Lett., Vol. 14, pp. 823-825, 1989,] or by illuminating the fibre through a periodic phase mask, as disclosed by K. O. Hill et.al in [xe2x80x9cBragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase-mask,xe2x80x9d Appl. Phys. Lett., Vol. 62, pp. 1035-1037,1993.] An FBG reflects light within a narrow bandwidth, centred at the Bragg wavelength, xcexB=2effxcex9, where neff is the effective refractive index seen by the light propagating in the fibre, and xcex9 is the physical period of the refractive index modulation. Outside the FBG bandwidth light will pass with negligible loss. If the fibre is birefringent the refractive index seen by light propagating in the two orthogonal polarisation eigenstates of the fibre, nx and ny will be different. Consequently, there will be two orthogonally polarised reflected spectra from the FBG with two Bragg wavelengths with a wavelength separation of xcex94xcexB=2Bxcex9, where B=nxxe2x88x92ny (nx greater than ny). Some birefringence can also be UV-induced through the writing of the grating.
It is known that the reflected Bragg wavelength from an FBG will change with any external perturbation which changes the effective refractive index seen by the propagating light and/or the physical grating period (fibre length), such as temperature and strain. By measuring the reflected Bragg wavelength, using for example a broadband light source and a spectrometer, an FBG can be used as a sensor for measuring such external perturbations. The bandwidth of the reflection spectrum from an FBG sensor is typically 0.1-0.3 nm (xcx9c10-30 GHz).
An external perturbation can also change the birefringent in the fibre, and hence change the wavelength separation between the two orthogonally polarised reflection spectra. This can be exploited to make a sensor where the wavelength splitting is a measure of an external perturbation, such as temperature, strain, or pressure, which directly or indirectly induces extra birefringence in the fibre. Such a sensor can also allow simultaneous measurement of two measurands, such as temperature and pressure/strain, by measuring both the wavelength splitting and the absolute wavelengths (or average wavelength). Various birefringent FBG sensors are disclosed in [U.S. Pat. No. 5,399,854 to Dunphy et.al., U.S. Pat. Nos. 5,591,965, 5,828,059, 5,869,835, all to Eric Udd, and U.S. Pat. No. 5,841,131 to Schroeder and Udd), and in [Sudo, M. et.al., xe2x80x9cSimultaneous measurement of temperature and strain using PANDA fiber gratingxe2x80x9d, Proc. 12th International Conf. on Optical Fiber Sensors,xe2x80x9d p. 170-173, 1997]. Interrogation of birefringent FBG sensors has been based on using sensors with sufficient birefringence to cause a splitting of the two reflection peaks which can be resolved by the interrogating spectrometer.
It is known that one or several reflected FBG sensor wavelengths can be measured using a broadband source, for example an edge-light-emitting diode (ELED) or a superfluorescent fibre source (SFS), in combination with a tuneable optical filter, for example a piezoelectric transducer (PZT) tuneable fibre Fabry-Perot filter [Kersey, A. D. et.al., xe2x80x9cMultiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filterxe2x80x9d, Optics Letters, Vol. 18, pp. 1370-1372, 1993], or alternatively a tuneable laser source [U.S. Pat. No. 5,401,956 (Mar. 28, 1995)], provided the source spectrum covers all possible FBG sensor wavelengths. To obtain accurate, repeatable wavelength measurements with these techniques one can use a reference scheme based on the use of a fixed Fabry-Perot filter and a reference FBG with separate detector channels [Norwegian patent No 307357]. There are also various other techniques for wavelength interrogation of FBG sensors with broad band sources, such as edge filtering, interferometric detection and direct spectroscopic detection [Kersey, A. D., et.al., xe2x80x9cProgress towards the development of practical fiber Bragg grating instrumentation systems,xe2x80x9d Proc. SPIE, Vol. 2839, 1996].
With standard low-birefringent FBG sensors, inherent or induced birefringent can result in measurement errors caused by the corresponding splitting of the orthogonally polarised FBG reflection spectra. Unwanted birefringence can for example be induced when FBG sensors are embedded in a composite structure. Normally, this splitting is smaller than the reflection bandwidth of the FBG. In an interrogation system with a partly polarised source such as an ELED or a polarised tuneable laser, and/or polarisation dependent components, the randomly varying birefringence in the lead fibres between the readout instrumentation and the sensors will cause variations in the relative reflected power in each of the two orthogonal FBG spectra, and hence variations/errors in the measured Bragg wavelengths. Polarisation scrambling techniques have been demonstrated which reduces these birefringence induced measurement errors [Ecke, W., et.al., xe2x80x9cImprovement of the stability of fiber grating interrogation systems using active and passive polarization scrambling devicesxe2x80x9d, Proc. 12th International Conf. on Optical Fiber Sensors,xe2x80x9d p. 484-487, 1997]. Alternatively, one can use an unpolarised source such as an SFS and a system with no polarisation dependant components. However, if the wavelength splitting due to birefringence in the FBG sensor occasionally becomes large enough to significantly change the spectra shape of the reflected spectrum or cause two resolved separate peaks this can give problems for the readout instrumentation and give erroneous results.
Birefringent FBG based two polarisation fibre laser sensors disclosed with a distributed feedback (DFB) fibre laser in the Norwegian patent No. 302441 to J. T. Kringlebotn, are attractive for high resolution measurements of birefringence inducing measurands. The laser light at the two orthogonally polarised eigenstates of the laser are mixed in a detector, generating an electrical beat frequency which is a measure of the birefringence induced in the laser sensor by the measurand.
The main object of the invention is to provide a method for accurate and high-resolution measurements of the orthogonally polarised maximum and minimum reflected Bragg wavelengths from fibre Bragg grating (FBG) sensors with birefringence, using a Bragg wavelength readout system based on a broadband source or alternatively a tuneable laser source. The aim is to provide a readout system which can measure birefringence-induced splitting of Bragg wavelengths with high resolution and accuracy (ca. 1 pm), independently of the magnitude of the splitting. A splitting larger than the FBG bandwidth will hence not be required.
A second objective is to provide a means for eliminating errors in FBG sensor measurements cause by the grating birefringence in combination with an interrogation system with a partly or fully polarised source and/or polarisation dependent components, independently of the magnitude of grating birefringence and degree of source polarisation.
A third objective is to provide a means for eliminating signal fading and optimise the signal amplitude of birefringent FBG based two polarisation fibre laser sensors.
The object of the invention is achieved with a method having features as stated in the characterising part of claim 1. Further features are stated in the dependent claims. The main part of the invention comprises the use of an electrically controllable fibre optic polarisation controller in combination with an FBG wavelength readout instrumentation system to measure the wavelength splitting of one or several wavelength multiplexed birefringent FBG sensors. The polarisation controller is either i) operated in a scanning mode to cover a wide range of polarisation states in a certain time period, including the two orthogonal polarisation states corresponding to the minimum and maximum Bragg wavelengths or ii) operated in a tracking mode with electrical feedback from the receiver of the instrumentation system to change the polarisation states in order to track the minimum and maximum Bragg wavelengths of each FBG sensor. The scanning mode can either use random scanning or a programmed sequential scanning to cover a grid of polarisation states on the Poincarxc3xa8 Sphere. The measured minimum and maximum wavelengths provide a measure of the measurand induced birefringence, independently of the magnitude of this birefringence. The invention can in combination with a wavelength read-out unit with a proper wavelength reference system provide absolute measurements of both orthogonal Bragg wavelengths, which enables simultaneous measurements of two independent measurements, such as temperature and pressure.
The invention can also be used to eliminate errors in FBG sensor measurements caused by the grating birefringence in combination with an interrogation system with a partly or fully polarised source and/or polarisation dependent components, independently of the magnitude of grating birefringence and degree of source polarisation.
Finally, the method provides a means for eliminating signal fading and optimise the signal amplitude of one or several birefringent FBG based two-polarisation fibre laser sensors.